Diode lasers are semiconductor lasers. A plurality of these lasers is typically produced by wafer-level processing, after which the wafer is diced into the individual lasers. In edge-emitting type of lasers the light propagates in the plane of the semiconductor wafer. The intersection of the gain medium of the laser, perpendicular to the propagation of the light, is typically rectangular shaped. The height is typically of the order of 1 μm. The width of the gain medium is determined by the application of the lasers: for single-spatial mode lasers, the gain medium has to be very narrow, while for high power applications the gain medium is made much wider (e.g. 20 times the height of the gain medium). In the case of a wide gain medium there can be several spatial modes present in the widest direction of the laser gain medium. For high-brightness laser projection based on a 2D imager, an excellent beam quality per laser is not required, and as such diodes with a relatively wide gain medium and increased power per emitter are preferred.
The direction of the width of the gain medium is typically called slow axis, while the direction of the height of the gain medium is typically called fast axis.
The shape of the diode's gain medium has an important influence on the beam properties of the diode laser. First of all the beam profile for high power diode lasers will often have an irregular shape in the slow axis, due to the presence of the multiple spatial modes: several maxima in the light intensity of the far field of the laser can be observed in the direction of slow axis. This is caused by the fact that due to the width of the gain medium, the light is less confined in space along the slow axis.
In addition, diffraction will cause a difference in the beam divergence. In the fast axis the beam will be more divergent than in the direction of the slow axis. The different divergence is caused by diffraction: a smaller aperture will result in larger diffraction angles. As such the widest direction of the laser's gain medium corresponds to the narrowest angular distribution. The divergence of the lasers can be 25° in one direction and 50° degrees in the other direction. Lasers are typically combined with collimation lenses to change the diverging bundle into a parallel bundle. The difference in divergence between fast and slow axis then translates into a larger beam waist in the fast axis then the slow axis after collimation.
This asymmetry is an important parameter for the optical design of high-brightness laser projectors, where the étendue of the system has to be managed accurately and any étendue loss has to be avoided, or at least minimized. To achieve the maximum brightness the maximum amount of individual laser diodes has to be combined within the available étendue of the system.
After dicing, the lasers are packaged, for example into a TO9 can, or other type of typically standardised package (e.g. also smaller size TO-can type packages exist: e.g. TO38, which is only 3.8 mm in diameter). FIG. 1 shows a picture of the TO9 package. The package is composed of a metal plate, which serves as the cooling surface for the laser. On this cooling plate a metal housing is mounted to protect the laser. The light is emitted in a direction perpendicular to the cooling plate (upwards in the left pane of FIG. 1) and leaves the TO9 can through a window in the housing. There are typically 2 or 3 pins on the back side of the cooling plate, which are used for driving the lasers. The problem with these pins is that they will penetrate the cooling surface on which the lasers are to be positioned: the heat dissipated in the laser is transferred to the cooling plate, which is the bottom of the package. Then this heat has to be removed by a cooling system. As such the cooling plate of the lasers will have to be mounted in close contact to a heat sink. This heat sink can be cooled by air or by a liquid cooling system. Cooling lasers is not trivial, as the lasers should typically be operated at temperatures around room temperature. In addition it is preferred to tightly control the temperature of the lasers, as temperature variations result in variations in the emitted optical power and in the wavelength of the lasers. Therefore the lasers are designed for and operated at a well-defined temperature; e.g. 25° C.
The length of the pins on the TO-9 can is also limited. If a direct connection with the driver printed circuit board is to be made, this significantly limits the thickness of the heat sink.